The performance of turbomachinery, such as a centrifugal compressor for a gas turbine engine, is partially dependent upon the quality of flow that such turbomachinery receives. In particular, it is important to minimise pressure loss and distortion of flow that enters the impeller of such centrifugal compressors in order to maximise efficiency and minimise noise.
Proper inlet design is a particular challenge for a gas turbine engine that has a radial inlet duct that couples airflow to an axial compressor impeller inlet by way of an inlet plenum that receives the airflow from the inlet duct and guides it to the compressor inlet plane. Such an arrangement is common for an aeronautical auxiliary power unit due to space constraints.
In order to keep flow-distortion at the plenum outlet to the compressor impeller to a minimum, the inlet plenum may comprise one or more inlet guide vane passages to help guide the airflow. Such guide vanes offer no barrier to ingestion of foreign matter in the airflow that may damage the compressor impeller, so such an arrangement also may comprise a foreign object damage (FOD) barrier or screen positioned in the airflow upstream of the guide vanes to prevent the ingestion of foreign matter by the compressor impeller. Such a FOD barrier or screen typically comprises a wire screen placed within the interface between the inlet duct and the inlet plenum.
A FOD barrier or screen of this type typically exhibits stability and integrity problems that make it unreliable at best and the source of FOD itself at worst. Consequently, an alternative inlet design utilises an inlet plenum that comprises a perforated inlet (PI) baffle. The PI baffle comprises a generally cylindrical baffle within the inlet plenum that circumscribes the generally annular inlet of the compressor. The airflow must pass through a plurality of apertures or perforations in the baffle, each aperture small enough to block foreign matter within the airflow greater than a critical size that may cause FOD to the compressor impeller.
However, the generally small wall thickness of a PI baffle offers almost no flow guidance compared to guide vanes. This generally results in significant flow distortion of the airflow to the compressor inlet and significant flow pressure losses across the apertures or perforations in the PI baffle. Choosing different sizes, shapes and distributions of the apertures or perforations along the perimeter or surface of the PI baffle could possibly reduce such flow distortion, but production and verification of structural integrity for such a PI baffle would be difficult and costly.